This invention relates to a cyclic process of hydrogen peroxide production.
This cyclic process involves hydrogenating an organic solution containing quinone into hydroquinone, then oxidizing this hydroquinone into quinone hydroperoxide with oxygen or air, and then decomposing this hydroperoxide with water into an organic solution containing regenerated quinone. This latter solution is then reintroduced into a new cycle: hydrogenation, oxidation and extraction.
In practice, the organic solution of quinones, called a shuttle solution or working solution, circulates in a series of three apparatus: an hydrogenator, an oxidizer and a liquid-liquid extraction column. The constitutive material of these apparatus is generally aluminum, although stainless steel is sometimes used.
As in most syntheses performed in an organic medium, secondary reactions occur in addition to the main reaction. Here, then, the secondary reactions occur in each of the three phases of the process and the overall yield of the process is the product of the partial yields of each operation. In the two last phases, oxidation and extraction, the presence of peroxides and oxygen causes these secondary reactions to be essentially oxidation reactions of the constitutive elements of the working solution and of decomposition of the perioxides.
Also decomposition of the peroxides causes a more intense corrosion of the metal forming the reactor walls and in particular aluminum. This corrosion can also induce other secondary reactions. Thus, from the earliest times of the process, inhibitors of hydrogen peroxide decomposition and aluminum corrosion have been added to the extraction water. Inhibitors most currently used are sodium pyrophosphate as a stabilizer of the H.sub.2 O.sub.2 decomposition and ammonium nitrate as a passivator of the aluminum.
In regard to the oxidizer, the apparent absence of an aqueous phase and the dessication due to the flow of hot air (40.degree. to 60.degree. C.) that travels through it, could lead to the belief that it is protected from corrosion and decomposition. Practice of the process shows that this is not the case; the presence of inorganic and organic salts, principally of sodium, causes a concentrated saline solution to deposit on its wall and causes a corrosion of the metal linked to the decomposition of the peroxides.
It is known, particularly from French Pat. No. 1,405,861, that addition of a hydrogen peroxide stabilizer and of a passivator in the oxidizer offers an advantage. According to this teaching, there is continuously introduced at a low delivery rate an aqueous solution containing sodium pyrophosphate and ammonium nitrate in the organic solution going into the oxidizer and also leaving the oxidizer to protect these accessory installations placed between the oxidizer and extractor.
Use of pyrophosphate, both in the oxidizer and extractor, therefore represented a very important improvement, the most remarkable feature of which was the elimination of alumina gel generated in the aqueous H.sub.2 O.sub.2 solution, a gel causing the rapid clogging of the filtration barrier placed downstream from the extraction of this solution. With pyrophosphate in the solution, aluminum phosphate is generated in place of the aluminum gel and such gel is replaced on the filters by a granular deposit of the aluminum phosphate, and in much less abundant qualities as the corrosion is greatly reduced.
However, the pyrophosphate ion, P.sub.2 O.sub.7.sup.4-, has two major drawbacks:
1. It hydrolizes into orthophosphate ion, PO.sub.4.sup.3-, much less active than the pyrophosphate ion, an hydrolysis that is faster the more acid the medium.
2. The complex that is formed with the Al ion is very insoluble in the vicinity of pH2, and even less so the more the H.sub.2 O.sub.2 concentration of the solution increases.
These drawbacks are not very apparent when the process is used with a relatively low productivity, i.e., when the concentration in "H.sub.2 O.sub.2 equivalent" of the organic solution is from 5 to 9 g/liter.
Actually, H.sub.2 O.sub.2 is intrinsically acid and its aqueous solutions have a pH that is lower the higher the concentration; as there is an H.sub.2 O.sub.2 concentration equilibrium between the oxidized organic solutions and the aqueous solutions contained in the oxidizer and extractor, an increase in the H.sub.2 O.sub.2 equivalent of the organic phase results in a reduction of the pH in the aqueous phase. For example, an H.sub.2 O.sub.2 equivalent of 10 to 12 g/liter in organic solution is in concentration equilibrium with a 650-700 g/l aqueous solution whose pH can then only be below 3; this results in an accelerated hydrolysis of the pyrophosphate ion and because such ion must be kept in excess (by possible additions), the aluminum pyrophosphate can precipitate by the joint effect of the H.sub.2 O.sub.2 concentration and the pH. In practice, this insolubility of the pyrophosphate-aluminum complex leads to the following drawbacks.
In the extraction column, and particularly at the foot of the column, where the hydrogen peroxide is most concentrated and the pH is minimum, adhering deposits of aluminum pyrophosphate are formed. These deposits are particularly present on the walls and glass peepholes where they impede visibility. These adhering deposits also appear on the perforated trays of the column, especially on the first plate, and in particular at the periphery of the perforations. The presence of these deposits reduces the delivery of the working solution, sooner or later causing stoppage of the installation and requiring cleaning of the trays and peepholes of the extraction column.
Downstream from the extractor there is noted the existence of adhering deposits of the pyrophosphate-aluminum complex on the vaporizers for concentrating hydrogen peroxide by distillation; and flocculates appear in the storage tanks of the concentrated hydrogen peroxide solutions. Also, upstream from the extraction phase, there is noted the appearance of adhering deposits on the walls of the oxidizer, and the exchangers and condensers placed between the oxidizer and extractor. These deposits are particularly damaging on aluminum, because, formed in the presence of a concentrated acid aqueous phase of H.sub.2 O.sub.2 they provide sites for corrosion under the deposits, with the formation of cracks that often become deep and serious.